1. Field of the Invention
The present invention relates to a ceramic film having one-dimensional through channels (hereinafter sometimes referred to as a one-dimensional through channel ceramic film), and to a method for manufacturing this film. More particularly, the present invention relates to a ceramic film with a variety of compositions and with one-dimensional through channels, which can be utilized favorably as a separation membrane for molecules and fine particles or as a carrier membrane for a catalyst with high activity or for a catalyst with a large specific surface area, and to a method for manufacturing this film efficiently and with good reproducibility. The one-dimensional through channel ceramic film of the present invention is useful as a means for separating a variety of molecules and fine particles or in chemical reaction control because it exhibits a nanometer-size separation function, a catalyst function, and a catalyst carrier function.
2. Description of the Art
Development is underway for ceramic porous materials and ceramic filters that have excellent heat and corrosion resistance when used to separate molecules or fine particles contained in high-temperature gases and corrosive liquids, or as carriers for high-activity catalysts or for catalysts that can be used at high temperatures of about 1000.degree. C. A method in which a ceramic raw material powder is sintered, and the open pores (through channels that open onto the surface of the ceramic material) formed in the course of the densification of the ceramic material are utilized in their as-formed state (Japanese Laid-Open Patent Application 7-8729) is often employed as a method for producing a porous ceramic or a porous ceramic membrane.
Nevertheless, a ceramic or ceramic membrane produced in this manner tends not to have pores of uniform size, and it was difficult to produce pores on the nanometer order with good reproducibility. Another problem was that pores closed up within the ceramic matrix (hereinafter sometimes referred to as closed pores), which did not contribute to the separation function or catalyst function, accounted for a considerable percentage of the total.
A porous silica has been produced by a process combining a sol-gel method with spinodal decomposition in an effort to produce with good reproducibility a porous material that has uniform pore size on the nanometer order and that contains almost no closed pores (Nakanishi et al., "Ceramic Transactions, Porous Materials," The American Ceramics Society, 51-60 (1992) ), but in this case the silica phase is formed with a reticulated texture, and the shape of the pores is irregular and the direction thereof random.
If the shape of the pores was thus irregular and the direction thereof random, then when the product was used as a ceramic filter, for example, there was a problem in that a pressure loss occurred because of the large quantity of the ceramic portion that scattered the molecules or particles moving in the permeation direction and hindered their movement.
In an effort to solve the above problems, there have been attempts at developing a ceramic film having a nanometer-size pore diameter and also having pores that go through one-dimensionally (hereinafter sometimes referred to as one-dimensional through channels). A typical example of this is a porous alumina film produced by the anodic oxidation of aluminum (Japanese Patent Publication 6-37291). A film produced by the anodic oxidation of aluminum offers the advantage of considerable heat and corrosion resistance because the pore diameter can be controlled within a range of from several nanometers to several tens of nanometers by varying the conditions under which the film is produced, and because the film is made up of alumina. However, a drawback was that in the case of a porous alumina film produced by anodic oxidation, because of the fundamental limitations imposed by the use of anodic oxidation of an aluminum foil or an aluminum sheet of some thickness in an electrolytic solution, the obtained film was limited to amorphous alumina that was only stable near room temperature, and an aluminum sheet was the only choice for the substrate.
A method in which a porous alumina film manufactured by anodic oxidation is used as a casting mold to transfer a pore pattern to a polymer membrane has been tried as a method for compensating for the above drawbacks to a porous alumina film produced by anodic oxidation (Masuda et al.; Japan Ceramics Society, preprints from Spring 1995 Conference, p. 485, 3F4 01).
In this case, however, the material to which the structure of the porous alumina film produced by anodic oxidation is transferred is limited to an organic material, and another drawback is the difficulty of applying a membrane to which the film structure has been transferred over another substrate to produce a new ceramic film composite.